1. Field of the Invention
Embodiments of the present invention generally relate to position monitoring and more particularly to position monitoring using resonant circuits and fiber optic delay lines.
2. Description of the Related Art
Position sensors are used in a wide variety of automated manufacturing applications. For example, in integrated circuit (IC) fabrication, position sensors may be used to determine the precise location of a robot arm, whether a chamber door is open or closed, whether a silicon wafer is properly centered on a wafer handler, and the position or status of many other moving or movable objects. It is not uncommon for an IC fabrication facility processing cluster tool to utilize hundreds of position sensors to monitor the position or status of various objects and associated processes. Due to this high number, the sum cost of the individual sensors may be significant. Therefore, it may be desirable to keep the cost of individual sensors as low as possible. However, while keeping the cost low, it may also be desirable for the sensors to maintain a high level of performance (e.g., response time, sensitivity, etc.), a high level of reliability, and a low level of complexity.
One common type of sensor used for position and status monitoring utilizes fiber optics to detect the position or presence of an object. Typically, a fiber optic position sensor uses an adjacent pair of fiber optic lines, one to carry light from a remote source to an object or target whose position or motion is to be sensed, and the other to receive the light reflected from the object and carry it back to a remote photo sensitive detector (a photo-detector). Basically, the sensors work on a simple principle: if the reflected pulses are detected, the object is present (i.e., within a field of view of the photo-detector).
However, traditional sensors using fiber optics may be susceptible to optical noise, which may affect their reliability. For example, if a photo-detector is exposed to light pulses from sun glints, optical flashes, or florescent light, traditional sensors may provide false position information. One approach to reducing an amount of optical noise that reaches the photo-detector is to use optical filters to filter out wavelengths outside a range of the optical pulse generated to illuminate the object. However, optical filters add additional cost to the sensor, and may be cost prohibitive in certain applications.
Another approach to compensate for optical noise is signal validation through confirmation of a number of optical pulses received by a photo-detector. For example, the sensor may include circuitry to confirm a spacing between multiple pulses is equal and/or that the width of the multiple pulses is consistent. The sensor may produce an indication the object is in the field of view of the sensor only if the pulse width and/or spacing is confirmed (i.e., that uniform pulses are not consistent with typical optical noise). However, the additional circuitry required for signal validation may also add additional cost, as well as complexity, to the sensor. Also, the multiple pulse validation may tend to reduce determinism in the detection.
Yet another approach to compensate for noise is to utilize automatic gain control (AGC) to detect an optical signal a threshold level above a predetermined noise floor. Besides circuitry required for automatic gain control, this approach also requires circuitry to control current to the photo-detector, which may also add additional cost and complexity to the sensor. Further, sensors employing automatic gain control typically have a limited range of approximately 4 inches, which may limit their utility in some applications.
Other types of position sensors utilize linear effects detectors, which tend to have poor resolution and slow response times. Some sensing systems utilize an array of charge-coupled devices (CCDs) to monitor position or status of an object. These systems tend to be relatively expensive and require complex optics. Further, CCD systems typically require a great deal of processing overhead (e.g., a dedicated computer or controller), which adds further cost and complexity.
Therefore, a need exists for an improved method and apparatus for sensing position and/or status of an object that is immune to noise and low in cost.
The present invention generally provides methods, apparatus and systems for sensing position information of an object utilizing one or more fiber optic delay lines and a resonant circuit.
One embodiment of a method generally includes illuminating the object with an optical pulse source and supplying a first optical pulse to a photo-detector, causing a resonant circuit formed by the photo-detector and an inductor to generate a resonant signal. The method also includes supplying at least a second optical pulse reflected from the object to the photo-detector causing a change in the resonant signal, monitoring the change in the resonant signal, and determining a position of the object based on the monitored change in the resonant signal. A velocity and direction of travel of the object may also be determined from successive position measurements.
One embodiment of an apparatus generally includes a means for illuminating the object and a resonant circuit formed by a photo-detector and an inductor. The apparatus also includes a first fiber optic line to supply a first optical pulse to the photo-detector, causing the resonant circuit to generate a resonant signal and at least a second fiber optic line to supply a second optical pulse reflected from the object to the photo-detector causing a change in the resonant signal. The apparatus also includes control circuitry adapted to monitor the resonant signal and determine a position of the object based on the resonant signal. The first fiber optic line may supply the first optical pulse directly from the illuminating means. Alternatively, the first fiber optic line may be positioned to receive an optical pulse reflected from the object and supply the reflected optical pulse as the first optical pulse.
Another embodiment of an apparatus generally includes a means for illuminating an object having an aperture formed therein and a resonant circuit formed by a photo-detector and an inductor. The apparatus also includes one or more fiber optic delay elements, each having a first fiber optic line to supply a first optical pulse passing through the object aperture to the photo-detector, causing the resonant circuit to generate a resonant signal and a second fiber optic line to supply a second optical pulse passing through the object aperture to the photo-detector causing a change in the resonant signal. The apparatus also includes control circuitry adapted to monitor the resonant signal and determine a position of the object based on the resonant signal.
One embodiment of a system generally includes a target attached to the object, a means for illuminating the target, and a resonant circuit comprising a photo-detector and an inductor. The system also includes a first fiber optic line to supply a first optical pulse to the photo-detector, causing the resonant circuit to generate a resonant signal, and at least a second fiber optic line to supply a second optical pulse reflected from the target to the photo-detector causing a change in the resonant signal. The system also includes control circuitry adapted to monitor the resonant signal and determine a status of the object based on the resonant signal.